Base Station Apparatus and Mobile Communication System

ABSTRACT

Disclosed is a handover method in radio communication. A base station apparatus monitors the receiving state of a terminal, detects, based upon the receiving state, whether the terminal is one in which there is a possibility that loss of a call or a decline in quality will occur or one in which loss of a call or a decline in quality has occurred, and hands over the terminal to another base station apparatus having a carrier frequency different from that of the present base station apparatus.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.11/247,649 filed Oct. 11, 2005, now pending, which is an continuation ofInternational Application PCT/JP03/07443 filed on Jun. 12, 2003, nowpending, the contents of which are herein wholly incorporated byreference.

BACKGROUND OF THE INVENTION

This invention relates to a base station apparatus and mobilecommunication system. More particularly, the invention relates to a basestation apparatus and mobile communication system for dealing with lossof calls or decline quality as caused by a decrease in transmissionspeed, wherein when a required transmission speed for a certain terminalor service is no longer met or the required transmission speed is metbut without enough margin, the terminal is handed over to anotherfrequency being used at the same position (location) without theterminal moving, thereby solving problems such as loss of calls.

Although a W-CDMA system is described below by way of example, theinvention is capable of being implemented in mobile communicationsystems unless stated otherwise. That is, the present invention isapplicable to mobile communications as a whole and not just to W-CDMAsystems alone.

In a W-CDMA system, service areas (hexagonal areas) in a cellularconfiguration of the kind shown in FIG. 48 are formed, a radio basestation is deployed at the center and each service area is composed ofone or a plurality of sectors. FIG. 48 illustrates a three-sectorarrangement. A radio base station will be referred to simply as a basestation or Node B below.

Further, a plurality of frequencies (carrier waves) are assigned to eachsector. FIG. 49 illustrates a case where two frequencies have beenassigned to one sector. The frequency of a base station Node B1 in FIG.49 is assumed to be f1 and its service area is indicated by the solidline. The frequency of a base station Node B2 is assumed to be f2 andits service area is indicated by the dashed line. For the sake ofillustration, the solid and dashed lines are drawn offset from eachother although they may just as well overlap. In FIG. 49, one basestation, i.e., one Node B, is indicated for one frequency. However, oneNode B may be associated with two frequencies, as illustrated in FIG.50. In this specification, basically one transceiver TRX is shown to bedeployed per frequency. Accordingly, there are two cases, namely a case(FIG. 49) where one transceiver (one frequency) is provided for one NodeB, and a case (FIG. 50) where a plurality of transceivers TRX1, TRX2 areprovided for one Node B.

It should be noted that a terminal normally is capable of sending andreceiving using all frequencies employed in a W-CDMA system.

Handover

An operation in which a terminal UE1 moves from a cell CL1 of a basestation BTS1 to a cell CL2 of a base station BTS2 to thereby change thebase station that is the connection destination, as illustrated in FIG.51, is referred to generically as handover. Handover based upon movementis well known and, depending upon the particular method, is classifiedinto soft handover, hard handover, different-frequency handover and cellchange. Simply “handover” will be used below.

Handover in a system such as the conventional 3GPP Release 99 and PDC(Personal Digital Cellular) is implemented under the initiative of aRadio Network Controller (RNC), which is above the base stationhierarchically. That is, the RNC manages movement of terminals andcontrols handover between cells, between sectors, between differentfrequencies and between different systems. Specific examples of controlare designating a handover-destination base station, designating theestablishment of a radio link to a handover-destination base station anddesignating the re-establishment of a radio link to a terminal UE. Thebase station (Node B) at this time executes processing in accordancewith the designation made by the RNC. Further, ordinary handover iscarried out as the terminal moves.

HSDPA (High Speed Downlink Packet Access)

In mobile communication such as W-CDMA, data communication is performedusing packets. In the case of W-CDMA, specifications are being reviewedin the 3GPP (3^(rd) Generation Partnership Project) and packetcommunication is being performed between radio base stations andterminals (mobile telephones, etc.) using protocols that have beendecided by the project.

In 3GPP at the present time, the HSDPA (High Speed Downlink PacketAccess) scheme is being studied in order to perform packet communicationat higher speeds. This is a technique for the purpose of adopting a highspeed of 2 Mbps for packet communication on the downlink (communicationfrom the base station to the terminal). As mentioned above, HSDPA isbeing studied with a view to implementing standardization in Release 5,which is a 3GPP specification. The major changes in HSDPA in comparisonwith 3GPP Release 9 of the conventional specifications are thecomposition of the radio channels, retransmission control and theintroduction of a scheduler. The composition of radio channels will bedescribed below in simple terms and a scheduler that is directly relatedto the present invention will be described as well.

FIG. 52 is a schematic view of the configuration of an HSDPA system. Aradio access system in 3GPP comprises an RNC (Radio Network Controller)1, a Node B (base station) 2 and UEs (User Equipment: terminals) 3. TheRNC is connected to a CN (Core Network) 4.

With HSDPA, a {circle around (1)} HS-DSCH (High Speed-Downlink SharedChannel) in a wired downlink section and a {circle around (2)} HS-PDSCH(High Speed-Physical Downlink Shared Channel) in a wireless downlinksection are used as packet-data transmission channels CH. That is,HS-DSCH and HS-PDSCH are channels exclusively for the downlink and areshared by a plurality of UEs. They transmit packets that have beenencoded as by turbo encoding.

In a wireless downlink section, a {circle around (3)} HS-SCCH(High-Speed Shared Control Channel) is set up as a high-speed controlchannel, and control information for allowing the plurality of UEs toreceive packet data on the HS-PDSCH is transmitted. The controlinformation includes a user identifier (UEID: User Equipment Identifier)and various parameters (radio spreading code, modulation scheme,data-length information, etc.) for receiving data on the HS-PDSCH. TheHS-SCCH is shared by a plurality of the UEs.

Furthermore, in a wireless uplink section, a {circle around (4)}HS-DPCCH (High Speed Dedicated Physical Control Channel) is set up on aper-user basis. HS-DPCCH is a dedicated channel. This is a channel thattransmits a value, which indicates the number of receivable bits, fromeach terminal to the base station based upon reception conditions(whether or not a packet could be received without errors) and thereceiving state (the C/I, as one simple example). Notificationindicative of the former, namely the reception conditions, is referredto as ACK (notification of acknowledgement of reception) or NACK(notification of reception failure), and information indicating thelatter, namely the receiving state, is referred to as CQI (ChannelQuality Indicator).

Channels in addition to those mentioned above are {circle around (5)} DLAssociated DPCH (Downlink Associated Dedicated Physical Channel) and{circle around (6)} UL Associated DPCH (Uplink Associated DedicatedPhysical Channel). These channels are radio channels establishedindividually between each terminal and the base station. These arechannels used in association with the HS-PDSCH in particular among theDPCHs (Dedicated Physical Channels) employed in conventional Release 99.These channels will be abbreviated to DL A-DPCH and UL A-DPCH below.

ACK/NACK and Retransmission Control

With HSDPA, data retransmission control is exercised between the Node B2 and UE3. The UE3 reports ACK (notification of acknowledgement ofreception) or NACK (notification of reception failure) with respect tothe received data to the Node B 2 using the HS-DPCCH.

The flow of retransmission control is illustrated in FIG. 53, thestructure of the terminal UE in FIG. 54 and the structure of the basestation Node B in FIG. 55.

The terminal UE3 receives a packet, which has been transmitted by theabove-mentioned HS-PDSCH, using a radio unit 3 a, demodulates anddecodes the packet using a demodulator 3 b, performs a CRC check using aretransmission controller 3 c and verifies the packet receptionconditions (e.g., whether or not the packet could be received withouterrors). For example, in the event that no errors have been found, theterminal transmits ACK via a modulator 3 d and radio unit 3 e using theabove-mentioned UL HS-DPCCH, thereby requesting the base station Node Bto perform a new transmission. On the other hand, if an error is foundas a result of the CRC check, the terminal transmits NACK using the ULHS-DPCCH, thereby requesting the base station Node B to performretransmission. Retransmission is performed until error-free receptioncan be achieved, by way of example.

Meanwhile, the base station Node B receives UL HS-DPCCH by a radio unit2 a and demodulates and decodes the packet in a demodulator 2 b. Thebase station then extracts the ACK/NACK signal in an ACK/NACK extractionunit 2 c and performs retransmission control using a retransmissioncontroller 2 d. Specifically, in case of ACK, the retransmissioncontroller 2 d deletes a successfully transmitted packet that has beenstored in a transmit buffer 2 e. In case of NACK, the retransmissioncontroller 2 d retransmits an unsuccessfully transmitted packet, whichhas been stored in the transmit buffer 2 e, via a modulator 2 f andradio unit 2 g using HS-PDSCH. Such retransmission control is carriedout by a scheduler, which is described next.

Whether ACK or NACK is sent back changes depending upon whether or notreceive data contains an error, which depends upon the receiving stateof the terminal UE. As for the cause, factors that depend upon thestates of C/I or/and S/N or/and the traveling speed of the terminal aresignificant. Here C/I stands for Carrier/Interference and corresponds toS/N and SIR (Signal/Interference), C represents signal power and Iinterference power with respect to interference. This is an index of themagnitude of interference. It indicates that the smaller the C/I, i.e.,the greater the amount of interference, the greater the degradation ofthe receiving state.

Scheduler

In HSDPA introduced by 3GPP Release 5, the above-mentioned radiochannels and a scheduler function that is for deciding the order ofpacket transmission are added anew. In order to describe the scheduler,a description will also be rendered with regard to HS-PDSCH. Unlike theconventional DPCH, HS-PDSCH is not a radio channel provided individuallyfor a terminal that is a communicating party. That is, one HS-PDSCH, forexample, is time-division multiplexed and is used by one terminal or bybeing shared by a plurality of terminals.

FIGS. 56 (A) to 56 (D) are diagrams useful in describing a mechanism forreceiving packet data on the HS-PDSCH.

As shown at (A) of FIG. 56, a transmit cycle referred to as a “TTI”(Transmission Time Interval=2 ms) is set up on HS-SCCH. Controlinformation is transmitted in conformity with the TTI and received by aplurality of UEs (two UEs #0 and #1) only if control information to betransmitted exists. The data transmitted on the HS-SCCH includes a useridentifier (UEID: User Equipment Identifier) and various parameters(radio spreading code, modulation scheme, data-length information, etc.)for receiving data on the HS-PDSCH.

UE receives HS-SCCH data in all TTIs. For example, in slot #1 at (B) ofFIG. 56, UE #1 and UE #2 receive the HS-SCCH data simultaneously. EachUE refers to the UEID in the data and compares it with its own ID. Inthis case, the UEID of the HS-SCCH data in slot #1 is “UE #1”, andtherefore UE #0 discards the receive HS-SCCH data and UE #1 loads thecontrol data contained in the receive HS-SCCH data. UE #1 thenceforthextracts a parameter, which is for HS-PDSCH receive, from the controldata portion, and receives the packet data on the HS-PDSCH [(C), (D) ofFIG. 56].

Upon receiving data, the UE #1 refers to a “sequence number” containedin the data and checks to determine whether there is loss of data. In acase where all data could be received without error (without CRC error)and without loss of data, the UE #1 reports ACK to Node B using theHS-DPCCH. Further, if data has been lost or a CRC check error hasoccurred, then the UE #1 reports NACK to Node B using the HS-DPCCH.Operation is similar with regard to slots #2 to #5 and slots #7 and #8.The UE #1 receives packet data via the HS-PDSCH of slots #1, #4, and theUE #0 receives packet data via the HS-PDSCH of slots #2 and #3, slot #5and slots #7 and #8.

A scheduler executes scheduling management and retransmission controlfor deciding in which slot a packet should be transmitted and whichterminal should be assigned. FIG. 57 is a diagram illustrating thestructure of base station Node B that includes a scheduler. Herereference characters 2 h represent a scheduler, 2 i a handovercontroller, and 2 j a CQI extraction unit for extracting CQIinformation, which is information indicating the receiving state of aterminal, from receive data.

An example of operation of the scheduler 2 h will be described below.Depending upon the CQI reported from a terminal and the communicationservice content (quality of service QoS) of the data transmitted, thescheduler 2 h decides the order of data transmission suited for eachterminal and effects the transmission in this order. A specific exampleof the decision of an order will be given below. It should be noted thatwhat follows is a representative method and that the method is notlimitative in any way.

{circle around (1)} C/I Method

On the basis of the C/I, transmission is performed in order ofdescending C/I excellence. In case of HSDPA, a CQI of high value isadopted as an excellent C/I. There is a possibility that a terminal witha poor C/I will not be given an opportunity to transmit.

{circle around (2)} Round Robin Method

This is a method in which transmission is performed equally irrespectiveof the receiving state of the terminal.

{circle around (3)} Proportional Fairness Method

This is a method in which transmission time is equalized andtransmission is performed in order of descending C/I excellence.

Further, in addition to the methods cited above, weighting with regardto the traffic class (Streaming class, Conversational class, Interactiveclass and Background class), which will be described later, also isconceivable. These traffic classes are referred to generically as QoS(Quality of Service). Maximum speed (bit/sec) and minimum speed(bit/sec), etc., are defined as parameters in QoS. In particular, incase of the Conversational or Streaming class, quick response is soughtin view of the applications of these classes and the stipulation onminimum speed is severe. In cases where the minimum speed is notcomplied with, service may no longer be provided, service may besuspended and the quality of transmitted data may not be maintained. Asexamples that may readily be understood, frame advance may occur when amoving picture is transmitted, and audio or video may be interrupted.

{circle around (1)} Conversational class: This is a class in which asmall-delay quality is required in both directions (example: voice).

{circle around (2)} Streaming class: This is a class in which asmall-delay streaming service is required in one direction (example:distribution of real-time moving pictures).

{circle around (3)} Interactive class: This is a class that requires aresponse within a fixed period of time as well as a low error rate(example: a Web browser or server access).

{circle around (4)} Background class: This is a best-effort class of thekind that is implemented in the background (example: E-mail or ftp).

Problems of the Prior Art

Depending upon the propagation environment in which a terminal is placedor the traveling speed of the terminal, problems may arise. That is, thestipulation on minimum speed in the QoS of the transmitted service maynot be adhered to, a call may be lost in the midst of communication andquality may decline. This will be described below using a specificexample.

Assume that a certain terminal UE2 is receiving a service (transmissionof a moving picture) that requires quick response, and that thethroughput required is 2 Mbyte/sec. Assume that since the propagationenvironment has worsened and interference has increased (i.e., that C/Ihas deteriorated), it is necessary to repeat retransmission and theactual throughput (transmission speed) has become 1 Mbyte/sec. A problemwhich arises at this time is that frames of the moving picture are lost,motion of people, etc., becomes stiff and the moving picture becomes astill picture. In some cases service must be halted because the qualityof the moving picture is not maintained.

Prior art (JP01-274524A) in which the communication rate (throughput) ofa mobile terminal is measured to render a decision as to whetherhandover should be performed is available as art for preventing loss ofcalls and a decline in quality. However, this is not art in which aterminal having little margin is allowed to handed over to a basestation of a different frequency.

Prior art (JP07-240959A) in which handover from a terminal having littlemargin is performed based upon the reception level is available as artfor preventing loss of calls and a decline in quality. However, this isnot art in which whether handover will be performed or not is decidedbased upon the communication rate (throughput) of a mobile terminal ordelay time or transmission power, and handover is allowed to be made toa base station of a different frequency.

Prior art (JP10-136425A) in which handover is performed at a differentfrequency is available. However, this is not art in which handover isperformed upon detecting a terminal that is likely to experience loss ofa call or a decline in quality based upon the communication rate(throughput) of the mobile terminal, delay time or transmission power.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to detect a terminalthat is likely to experience loss of a call or a decline in quality andhand over this terminal to a base station having a different frequency,thereby preventing loss of calls and a decline in quality.

A base station apparatus according to the present invention monitors thereceiving state of a terminal, detects, based upon the receiving state,whether the terminal is one in which there is a possibility that loss ofa call or a decline in quality will occur or one in which loss of a callor a decline in quality has occurred, and hands over the terminal toanother base station apparatus having a different carrier frequency.

More specifically, a base station apparatus receives and demodulates asignal that has been transmitted from each terminal, extracts CQIinformation, which is indicative of the receiving state of the terminal,from the demodulated data, sets the size of data to be transmitted tothis terminal based upon the CQI information of this terminal,calculates transmission speed based upon the size of this transmit data,transmit time and receive time of this data, determines whether handoveris necessary for this terminal based upon a required transmission speed,which is decided by service quality QoS of the transmit data, and thecalculated transmission speed, and hands over this terminal to anotherbase station apparatus, which has a different carrier frequency, ifhandover is necessary.

In another example, the base station apparatus receives and demodulatesa signal that has been transmitted from each terminal, extracts CQIinformation, which is indicative of the receiving state of the terminal,from the demodulated data, sets the size of data to be transmitted tothis terminal based upon the CQI information of this terminal,calculates transmission delay time based upon the size of this transmitdata, transmission time and reception time of this data, determinesnecessity of handover for each terminal based upon a maximum allowabledelay time, which is decided by service quality QoS of the transmitdata, and the calculated transmission time, and hands over this terminalto another base station apparatus, which has a different carrierfrequency, if handover is necessary.

Thus, in accordance with the present invention, in a case where arequired transmission speed for a certain terminal or service is nolonger met or the required transmission speed is met but without enoughmargin, the terminal is handed over to another frequency being used atthe same position (location) without the terminal moving, thereby makingit possible to solve problems such as loss of calls.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a structural view of a first embodiment in which throughput(transmission speed) is calculated on a per-service basis and handoveris performed based upon the throughput;

FIG. 2 illustrates an example of a protocol in the first embodiment;

FIG. 3 is an example of a first processing flow according to the firstembodiment;

FIG. 4 is an example of a second processing flow according to the firstembodiment;

FIG. 5 is an example of a third processing flow according to the firstembodiment;

FIG. 6 is an example of a fourth processing flow according to the firstembodiment;

FIG. 7 is an example of a fifth processing flow according to the firstembodiment;

FIG. 8 is an example of a sixth processing flow according to the firstembodiment;

FIG. 9 is a structural view of the general form of the first embodiment;

FIG. 10 is a structural view of the first embodiment in a case where ahandover controller is incorporated in a scheduler;

FIG. 11 is a structural view of a case where two carrier waves(frequencies f1, f2) have been assigned to one Node B;

FIG. 12 is a structural view of a second embodiment;

FIG. 13 illustrates an example of a protocol in the second embodiment;

FIG. 14 is a first processing flow according to the second embodiment;

FIG. 15 is a second processing flow according to the second embodiment;

FIG. 16 is a third processing flow according to the second embodiment;

FIG. 17 is a structural view of a third embodiment;

FIG. 18 illustrates an example of a protocol in the third embodiment;

FIG. 19 is a first processing flow according to the third embodiment;

FIG. 20 is a second processing flow according to the third embodiment;

FIG. 21 is a structural view of a fourth embodiment;

FIG. 22 illustrates an example of a protocol in the fourth embodiment;

FIG. 23 is a first processing flow according to the fourth embodiment;

FIG. 24 is a second processing flow according to the fourth embodiment;

FIG. 25 is a structural view of a modification;

FIG. 26 is a first control processing flow of a base station;

FIG. 27 is a second control processing flow of a base station;

FIG. 28 is a structural view of a fifth embodiment;

FIG. 29 is a first control processing flow of a base station;

FIG. 30 is a second control processing flow of a base station;

FIG. 31 is a structural view of a sixth embodiment;

FIG. 32 is a first control processing flow of a base station;

FIG. 33 is a second control processing flow of a base station;

FIG. 34 is a structural view of a seventh embodiment;

FIG. 35 illustrates an example of a protocol in the seventh embodiment;

FIG. 36 is a first processing flow of a base station;

FIG. 37 is a second processing flow of a base station;

FIG. 38 is a structural view of an eighth embodiment;

FIG. 39 is a first processing flow according to the eighth embodiment;

FIG. 40 is a second processing flow according to the eighth embodiment;

FIG. 41 is a structural view of a ninth embodiment;

FIG. 42 is a first processing flow according to the ninth embodiment;

FIG. 43 is a second processing flow according to the ninth embodiment;

FIG. 44 is a first conceptual view of a tenth embodiment;

FIG. 45 is a second conceptual view of the tenth embodiment;

FIG. 46 illustrates a protocol in the tenth embodiment;

FIG. 47 is a processing flow according to the tenth embodiment;

FIG. 48 is a diagram useful in describing service areas in a cellularconfiguration;

FIG. 49 is a diagram useful in describing a case where two frequencieshave been assigned to one sector;

FIG. 50 is a diagram useful in describing an arrangement one Node B usestwo frequencies;

FIG. 51 is a diagram useful in describing handover;

FIG. 52 is a schematic view of the configuration of an HSDPA system;

FIG. 53 is a diagram useful in describing flow of retransmissioncontrol;

FIG. 54 is a structural view of a terminal UE;

FIG. 55 is a structural view of a base station Node B;

FIG. 56 is a diagram useful in describing a mechanism for receivingpacket data on an HS-PDSCH; and

FIG. 57 is a structural view of a base station Node B that includes ascheduler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (A) First Embodiment

FIG. 1 is a structural view of a first embodiment in which throughput(transmission speed) is calculated on a per-service basis and handoveris performed based upon this throughput, FIG. 2 illustrates an exampleof a protocol and FIGS. 3 to 8 illustrate examples of processing flow.FIG. 3 illustrates the processing flow of processing P1 in the protocolof FIG. 2, FIG. 4 illustrates the processing flow of processing P2, FIG.5 illustrates the processing flow of processing P3, FIGS. 6 and 7illustrate the processing flows of processing P4, and FIG. 8 illustratesthe processing flow of processing P5. Components in FIG. 1 identicalwith those of the prior art described in FIG. 57 are designated by likereference characters.

A case in which it is assumed that communication is performed betweenbase station Node B and terminals UE1 to UEn (not shown) and isimplemented by HSDPA of W-CDMA will be described below as one example.

First, as will be illustrated in FIG. 3, a terminal UEk measures orcalculates C/I or SIR by receiving a pilot channel CPICH (step 101).Based upon the result, the terminal estimates the receiving state (orpropagation environment) of the terminal UEk and calculates CQI (step102). The CQI is obtained by preparing a table of correspondence betweenC/I and CQI and finding it from the table. Next, the terminal encodesand modulates the CQI, places it on the HS-DPCCH and sends it back tothe base station Node B (step 103), as illustrated in FIG. 2.

At base station Node B, a radio receiver (not shown) receives HS-DPCCH,the demodulator 2 b demodulates and decodes the receive signal and a CQIextraction unit 2 j extracts CQI information from the decoded data (FIG.4; step 151). In this case, it is possible to identify from whichterminal the information was obtained by using a spreading code specificto the terminal.

Based upon the CQI reported from each terminal and the communicationservice content (quality of service QoS) of the transmit data, atransmit UE selector 11 of the scheduler 2 h decides the order of datatransmission suited for each terminal and inputs the data to thetransmit buffer 2 e (step 152).

A block size setting unit 12 calculates the transport block size(TrBlkSize) based upon the CQI (step 153) and inputs a terminal numberof interest and number of transmit bits (transport block size TrBlkSize)to the transmit buffer 2 e.

The block size setting unit 12 notifies a throughput calculation unit 13of the terminal number (UE number), which is the communicating party,the transport block size (TrBlkSize), the time of transmission and theservice quality QoS (or traffic class) of the transmit data (step 154).Since throughput is substantially defined as transmission speed,transmission speed will be abbreviated to throughput below.

The transmit buffer 2 e inputs data, which conforms to the transportblock size of the terminal, to the modulator 2 f, and the modulator 2 fencodes and modulates the data and transmits it to the terminal usingHS-PDSCH (see FIG. 2) (step 155).

As illustrated in FIG. 5, the terminal UEk that has received HS-PDSCHdetermines by a CRC check whether the received data contains an error(steps 201 to 203). If the data contains an error, the terminalconstrues that the data has not arrived and sends NACK back to the basestation Node B using HS-DPCCH (see FIG. 2; step 204). If there is noerror, the terminal sends back ACK similarly using HS-DPCCH (step 205).

At the base station that has received HS-DPCCH from the terminal UEk,the demodulator 2 b demodulates and decodes the received signal and theACK/NACK extraction unit 2 c extracts ACK/NACK from the demodulateddata, records the reception time and inputs the results to throughputcalculation unit 13 (steps 251 to 253). In case of NACK, the scheduler 2h performs retransmission control (step 254). In case of ACK, thethroughput calculation unit 13 checks the quality of service QoS (step255) and calculates throughput Tks for every quality of service QoSbased upon the transport block size TrBlkSize held at the time oftransmission, transmission time ts and reception time tr in accordancewith the following equation:

Tks=TrBlkSize/(tr−ts)

and inputs the throughput to handover controller 2 i (step 256).

Further, the throughput calculation unit 13 obtains a requiredthroughput Tkds that conforms to QoS and inputs this to the handovercontroller 2 i (step 257). The method of calculating the requiredthroughput Tkds involves preparing a table of correspondence between QoSand the required throughput and finding the required throughput Tkdsfrom this correspondence table.

Further, a throughput threshold value generator 14 inputs a throughputthreshold value Tksth, which conforms to the QoS set beforehand at ahigher layer, to the handover controller 2 i (step 258). The throughputthreshold value Tksth is a threshold value of throughput margin, whichis the difference between the actual throughput and desired throughput.

The handover controller 2 i calculates the difference Tks−Tkds betweenTks and the required throughput Tkds and compares the difference(Tks−Tkds) with the throughput threshold value Tksth of throughputmargin (step 259).

If Tks−Tkds<Tksth holds, then the controller adopts the terminal ofinterest as a handover candidate and calculates and storesδk=Tksth−(Tks−Tkds) (step 260). On the other hand, if Tks−Tkds>Tksthholds, then the controller does not adopt the terminal as a handovercandidate and takes no action.

The controller thenceforth executes the above-described processing withregard to all terminals (UE1 to UEn) currently connected (step 261) andselects a terminal UEm having the least margin with respect to thethreshold value in accordance with the processing flow of FIG. 7. Itshould be noted that since it is also possible to furnish a plurality ofservices to a terminal, throughput can be calculated for every QoS withrespect to one terminal and it is possible to determine the differencewith respect to the required throughput as well as the margin.

In FIG. 7, initialization is performed as δmax=0 (step 271) Next, afteri=0 is performed, i is incremented (steps 272, 273) and whether i hasexceeded a number n of handover candidates is checked (step 274). If i≦nholds, an ith candidate δi and δmax are compared in size (step 275). Ifδi≦δmax holds, control returns to step 273 and processing from this steponward is repeated. If δi>δmax holds, the operation δmax=δi isperformed, UEm=ith candidate is adopted (step 276), control returns tostep 273 and processing from this step onward is repeated.

On the other hand, if i>n is found to hold at step 274, the ithcandidate that has been stored is selected as the terminal UEm havingthe least margin with respect to the threshold value (step 277), thisterminal UEm is transmitted to the radio network controller RNC andhandover is requested (step 278).

The handover management unit of the RNC performs conventional well-knownhandover control from the base station Node B to the terminal UEm forwhich handover has been requested. As illustrated in FIG. 8, thishandover control includes selecting the handover-destination Node B2having a carrier frequency different from that of the present basestation (step 301), setting a radio link between thehandover-destination Node B2 and the UE (step 302), implementinghandover (step 303) and canceling the radio link between original NodeB1 and the UE (step 304)

By achieving handover to a base station of a different frequency basedupon the first embodiment, the following effects are obtained:

In a case where the terminal UEm has moved to a better transmissionenvironment, it becomes possible to perform communication without afailure such as loss of an image in the transmission of a movingpicture, by way of example.

The base station Node B is capable of alleviating the processing loadascribable to communication. Further, it is possible to level theprocessing load between base stations or frequencies. As a result, it ispossible to service new users.

A communication system (provider) may readily assure a communicationspeed with respect to a service (QoS). As a result, an appealing offerof high quality can be made to users.

In the first embodiment, CQI is calculated based upon C/I that has beenmeasured or calculated and the CQI is sent back to the base station.However, C/I may be sent back without using the CQI, and S/N may be sentback. Further, although transport block size TrBlkSize is decided in thebase station based upon this CQI, transmission may be performed usingTrBlkSize, which has been specified at the higher layer (RNC), withoutusing CQI.

Further, although data arrival/non-arrival is sent back using ACK/NACK,ACK/NACK need not be used if calculation of throughput is possible.

Further, it is possible to perform handover based simply upon whetherthe required throughput has or has not been met without determining themargin of throughput, i.e., in a case where Tks<Tkds holds. In thiscase, however, there is a possibility that a call will be lost andcommunication terminated.

The throughput threshold value Tksth may be provided from a higher layeror decided by the base station. Further, the throughput threshold valueTksth may be varied for every service and may be changed dynamicallybased upon the transmission environment.

Furthermore, since the required throughput indicates the time that canbe allowed in order to transmit certain data, it is equivalent to themaximum allowable amount of delay.

It is also possible to share the burden of processing between the RNCand base station as by using the RNC to make the comparison withthreshold value of throughput margin.

Further, it is possible to perform control by a higher-layer devicerather than by the RNC. By controlling and managing handover in an RNCof a higher layer, selection of the frequency of thehandover-destination is facilitated.

The transmit UE selector 11 that selects the terminal as the party tocommunications in accordance with the conventional method is adopted asa narrow-sense scheduler and portions inclusive of functional portionsfor retransmission control and for deciding transport block sizeTrBlkSize, AMC (Adaptive Modulation and Coding) and the like shall bereferred to as the scheduler 2 h.

FIG. 9 is a structural view of the general form of the first embodiment.Here the ACK/NACK extraction unit 2 c, CQI extraction unit 2 j, blocksize setting unit 12, throughput calculation unit 13 and throughputthreshold value generator 14 of FIG. 1 have been changed to aretransmission control information extraction unit 2 c′, a qualityinformation extraction unit 2 j, a transmit bit number setting unit 12′,a transmission speed calculation unit 13′ and a transmission speedthreshold value generator 14′, respectively.

In the embodiment of FIG. 1, the handover controller 2 i is not placedin the scheduler 2 h. As shown in FIG. 10, however, it can also beincorporated in the scheduler 2 h. Further, the first embodimentillustrates a case where one Node B is associated with one carrier wave,i.e., where one carrier wave is assigned to one Node B. However,plurality carrier waves can be assigned to one Node B. FIG. 11 is astructural view of a case where two carrier waves (frequencies f1, f2)have been assigned to one Node B. This is an arrangement in which atransceiver (scheduler and handover controller) is provided for eachcarrier wave, and it is just as if two Node Bs, each of which has onecarrier wave assigned thereto, exist. Furthermore, a handover managementunit 1 a in the RNC performs the handover control of FIG. 8.

(B) Second Embodiment

In the first embodiment, the base station Node B exercises handovercontrol, decides the necessity of handover and the terminal to be handedover and requests the radio network controller RNC to perform handover.However, all of this processing can be brought to the RNC.

FIG. 12 is a structural view of a second embodiment, FIG. 13 is anexample of a protocol in the second embodiment and FIGS. 14 to 16illustrate processing flow. Only portions that are different from thefirst embodiment will be described below. In the protocol of FIG. 13,processing P1 to P3 is the same as that of the first embodiment, andprocessing P41, P51 is different from that of the first embodiment. FIG.14 shows the processing flow of processing P41, and FIGS. 15, 16 showthe processing flow of processing P51.

In the second embodiment, the required throughput Tkds with respect toservice quality QoS, the actual throughput Tks for every UE and everyQoS and the throughput threshold value Tksth are all managed in unifiedfashion by the RNC, which is the higher layer, and whether handover isnecessary or not is determined. As a result, the determination as towhether or not handover is necessary and the selection of the frequencyof the handover destination are facilitated by taking into accountstatus of use and actual throughput with regard to a frequency beingused by a certain sector. This arrangement also has a high degree ofaffinity for conventional systems.

Only operation different from that of the first embodiment will bedescribed below.

In a base station Node B 21 having the frequency f1 as its carrier,processing for calculating actual throughput Tks with regard to acertain terminal UEk and required throughput Tkds is similar to that ofthe first embodiment (steps 251 to 256 in FIG. 14).

Next, the throughput calculation unit 13 and throughput threshold valuegenerator 14 of base station Node B 21 report the throughput Tks, therequired throughput Tkds and the throughput threshold value Tksth to theRNC (step 265). At this time the throughput calculation unit 13 alsoreports the UE number (k in this case) and QoS (or traffic class) to theRNC.

The handover management unit 1 a of the RNC exercises handover controlin accordance with the processing flow of FIG. 15. Specifically, uponreceiving the throughput Tks, the required throughput Tkds and thethroughput threshold value Tksth (steps 250 a to 250 c), the handovermanagement unit 1 a selects the terminal UEm that has the least marginor that does not meet the required throughput (steps 259 to 261) in amanner similar to that of the first embodiment (see the processing flowof FIG. 6).

Next, the handover management unit 1 a selects the handover destination(e.g., base station Node B 22, which has the different frequency f2 asits carrier) and performs handover in accordance with the processingprotocol of FIG. 16. It should be noted that step 300 is a step ofdeciding the handover terminal by processing identical with that of FIG.7, and that steps 301 to 304 are processing steps identical with thoseof FIG. 8 in the first embodiment.

Thus, in accordance with the second embodiment, effects similar to thoseof the first embodiment can be obtained. Furthermore, control of the RNCbecomes flexible and easier owing to the fact that the information forperforming handover can be managed in unified fashion.

(C) Third Embodiment

A third embodiment has a controller for every carrier frequency andexecutes decentralized processing of handover solely within the basestation.

FIG. 17 is a structural view of the third embodiment, FIG. 18 is anexample of a protocol in the third embodiment and FIGS. 19, 20illustrate processing flow. In the protocol of FIG. 18, processing P1 toP3 and P5 is the same as that of the first embodiment, and processingP42 is different from that of the first embodiment. FIGS. 19, 20 showthe processing flow of processing P42.

In the first and second embodiments, management and control of handoverare carried out at a higher layer (e.g., the RNC). In the thirdembodiment, however, control and management of handover are performed bythe base stations or by the transceivers within the base stations, asindicated by the arrow symbols between the handover controllers. Thatis, decentralized autonomous control is performed and not unifiedcontrol by the higher-layer apparatus.

Only operation that differs from that of the first embodiment will bedescribed below. It will be assumed that a terminal that has beenassigned to the transceiver (frequency f1) 21 within the base stationNode B will be handed over to the transceiver (frequency f2) 22 withinthe base station Node B.

In the transceiver 21, processing up to the selection of terminal UEmhaving the least margin with respect to a threshold value and thedecision to perform handover (steps 251 to 261 in FIG. 19, step 262 inFIG. 20) is similar to that of the first embodiment. Step 262 is aprocessing step of deciding the handover terminal in accordance with theprocessing flow of FIG. 7.

Next, the handover controller 2 i of transceiver 21 selects the handoverdestination from among base stations or transceivers within basestations that are transmitting a frequency that terminal UEm, which isto be handed over, is capable of receiving without moving (step 263 inFIG. 20). Assume that the destination is transceiver 22 in Node B. Thehandover controller 2 i then requests handover to the handovercontroller 2 i of this transceiver 22 (step 264).

The handover controller 2 i of transceiver 22 to which handover has beenrequested performs handover control with respect to the terminal UEm.

Thus, in accordance with the third embodiment, effects similar to thoseof the first embodiment can be obtained. In addition, according to thethird embodiment, it becomes possible to perform handover without theintermediary of the RNC, as a result of which the amount ofcommunication with the higher layer can be reduced. Further, since it ispossible for information to be exchanged directly, the time required forhandover can be shortened.

(D) Fourth Embodiment

In the first to third embodiments, the difference between the requiredthroughput and the actual throughput is provided with a threshold valueand handover is performed from the terminal of greatest degradation. Ina fourth embodiment, maximum allowable delay time that has been set foreach service is taken into consideration, the terminal that is moststrict concerning delay is selected and handover control is carried out.

FIG. 21 is a structural view of the fourth embodiment, FIG. 22 is anexample of a protocol in the 22 embodiment and FIGS. 23, 24 illustrateprocessing flow. FIG. 21 differs from the first embodiment of FIG. 1 inthat a delay time calculation unit 31 is provided instead of thethroughput calculation unit 13, and a maximum allowable delay timesetting unit 32 is provided instead of the throughput threshold valuegenerator 14. In the protocol of FIG. 22, processing P1 to P3 and P5 isthe same as that of the first embodiment, and processing P43 isdifferent from that of the first embodiment. FIGS. 23, 24 show theprocessing flow of processing P43.

It is assumed that the terminal UEk, which is similar to that of thefirst embodiment, is receiving a service q1 (QoS=q1), and that themaximum allowable delay time is tq1_max. It should be noted that themaximum allowable delay time is set in the maximum allowable delay timesetting unit 32 from the higher layer. The time required for certaindata to be transmitted to the terminal correctly is the transmissiondelay time. Let this be represented by tk,q1. As for the method ofcalculating the transmission delay time, the time at which the basestation transmitted the data to the terminal is stored by the delay timecalculation unit 31 of the scheduler 2 h. The delay time calculationunit 31 monitors the reception time of ACK sent back from the terminal,calculates the transmission delay time tk,q1 per unit amount of data inaccordance with the following equation based upon the transmitted amountof data (transport block size TrBlkSize) and the above-mentionedtransmit time ts and receive time tr:

tk,q1=(tr−ts)/TrBlkSize

and inputs the transmission delay time to the handover controller 2 i(steps 401 to 405 in FIG. 23). Next, the maximum allowable delay timesetting unit 32 inputs the maximum allowable delay time tq1_max, whichconforms to the quality of service QoS set in advance, to the handovercontroller 2 i (step 406).

In order to evaluate the actual transmission delay time with respect tothe maximum allowable delay time, the handover controller 2 i adopts theratio between the two as transmission delay time margin and performs acalculation according to the following equation (step 407):

$\delta_{k,{q\; 1}} = \frac{t_{q\; 1\_ \; {ma}\; x}}{t_{{k,{q\; 1}}\;}}$

This indicates that the larger δ_(k,q1), the greater the margin withrespect to the maximum allowable delay time, and the smaller δ_(k,q1),the less margin. In communication over a fixed period of time, thehandover controller calculates the transmission delay time allowancewith respect to all services to all terminals (step 408), finds theterminal UEk for which the value of margin is smallest by the processingof FIG. 24 and requests the RNC to perform handover control of theterminal UEk. By control the same as that of the first embodiment, theRNC exercises control to hand over the terminal to another base stationapparatus having a different carrier frequency.

Thus, handover is performed from the terminal having the smallesttransmission time margin. However, handover may be performed from aterminal having a large transmission time margin. This is because owingto movement of a terminal with a large margin, there is a possibilitythat there may be an improvement in a terminal that exhibited a smallmargin until it moved.

Thus, in accordance with the fourth embodiment, effects similar to thoseof the first embodiment can be obtained.

Modification

It is possible to perform handover control by combining the first andfourth embodiments. FIG. 25 is a structural view of a modification andFIGS. 26, 27 illustrate the processing flows of a base station.

The base station executes the base-station processing of the first andfourth embodiments in accordance with the processing flow of FIG. 26,calculates the transmission delay time margin δk,q1 (=t_(q1) _(—)_(max)/t_(k,q1)) and throughput margin δk [=Tksth−(Tks−Tkds)] of eachUEk and stores these margins.

If the transmission delay time margin δk,q1 and throughput margin δk ofall terminals are found, the handover controller 2 i refers to thethroughput margin δk and decides a first order of priority of handoverfrom a terminal that has experienced the most degradation (step 451).The handover controller 2 i further refers to the transmission delaytime margin δk,q1 to decide a second order of priority of handover froma terminal having a strict transmission delay time margin (step 452).

Next, the handover controller 2 i takes these two orders of priorityinto account to decide the terminal to be handed over (step 453). Forexample, points are assigned to the first order of priority.Specifically, rank 1 of the order of priority is assigned 20 points,rank 2 is assigned 19 points and rank L1 is assigned (20−L1+1) points.Similarly, rank 1 of the second order of priority with respect to themaximum allowable delay time is assigned 20 points, rank 2 is assigned19 points and rank L2 is assigned (20−L2+1) points. Assume now that therank of a certain UE with respect to the threshold value is m1, that therank with respect to the maximum allowable delay time is m2, and thatboth of these scores are multiplied together to obtain(20−m1+1)×(20−m2+1). This processing is applied to all terminals and theterminal having the highest score is adopted for handover.

If a terminal to be handed over is found, the RNC is requested toperform handover. In response, the RNC selects a base station apparatusNode B, which has a different carrier frequency, as the handoverdestination (step 454) and exercises control to hand over the targetterminal to this base station (step 455).

It should be noted that although the threshold value and the maximumallowable delay time are handled as being equivalent, it is permissibleto apply weighting. Further, in the manner described above, handover maybe performed from the terminal having the lowest score.

Thus, in accordance with this modification, effects similar to those ofthe first embodiment can be obtained.

(E) Fifth Embodiment

A fifth embodiment is one for controlling handover taking intoconsideration the margin and priority with regard to a threshold value.

FIG. 28 is a structural view of the fifth embodiment, and FIGS. 29, 30illustrate processing flows of a base station. FIG. 28 differs from thefirst embodiment of FIG. 1 in that a priority management unit 15 isprovided and in that the handover controller 2 i controls handovertaking priority into account.

The decision on priority (order of priority) includes the factors setforth below. In this embodiment the method of setting order of prioritydoes not particularly matter.

Priority Among Terminals

As a specific example, assume a case where a telephone that is easy toconnect to an outside line and a telephone that is difficult to connectto an outside line exist in an extension telephone system that has beenintroduced in an enterprise. That is, there is a ranking for everyterminal. The priority is assigned by the higher layer.

Priority Among Services (Between QoS's)

Ranking of the following kind is conceivable: For example, sincetransmission of a moving picture requires quick response, the priorityof such a transmission is raised. With regard to a transmission such asan ftp transmission in which transmission time is not a concern, thepriority of transmission is lowered. Further, even in one and the samemoving-picture transmission system, for example, priority need notnecessarily be the same. High and low priorities exist depending uponthe content. The setting of priority is performed at the higher layer.

Priority Based Upon Propagation Environment of Terminal

This relates to the scheduler described in the first embodiment. Aterminal is ranked depending upon whether the propagation environmentsuch as the S/I of the terminal is good or bad.

The setting of priority may be decided by the base station or by thehigher layer. In the illustration, order of priority conforming to theQoS has been set in the priority management unit 15 from the higherlayer.

As mentioned above, the QoS (service) is ranked. Consequently, a methodin which the terminal UE to be handed over is merely decided based uponthe throughput of every service, as in the first embodiment, is notnecessarily the best method. Accordingly, in the fifth embodiment, theterminal handed over is selected taking into consideration the marginand priority of throughput with respect to a threshold value.

The base station executes the base-station processing of the firstembodiment in accordance with the processing flow of FIG. 29, calculatesthroughput margin δk [=Tksth−(Tks−Tkds)] of each terminal UEk, obtainsthe priority Pk of each terminal and stores δk and Pk as a set.

If the throughput margin δk and priority Pk of all terminals are found,the handover controller 2 i, in accordance with the processing flow ofFIG. 30, refers to the throughput margin δk and decides a first order ofpriority of handover from a terminal that has experienced the mostdegradation (step 501). The handover controller 2 i further refers tothe priority 2 k to decide a second order of priority of handover from aterminal having a low priority (step 502).

Next, the handover controller 2 i takes these two orders of priorityinto account to decide the terminal to be handed over (step 503). Forexample, points are assigned to the first order of priority.Specifically, rank 1 of the order of priority is assigned 20 points,rank 2 is assigned 19 points and rank L1 is assigned (20−L1+1) points.Similarly, rank 1 of the order of priority with respect to priority isassigned 20 points, rank 2 is assigned 19 points and rank L2 is assigned(20−L2+1) points. Assume now that the rank of a certain UE with respectto the threshold value is m1, that the rank with respect to priority ism2, and that both of these scores are multiplied together to obtain(20−m1+1)×(20−m2+1). This processing is applied to all terminals and theterminal having the highest score is adopted for handover.

If a terminal to be handed over is found, the RNC is requested toperform handover. In response, the RNC selects a base station apparatusNode B, which has a different carrier frequency, as the handoverdestination (step 504) and exercises control to hand over the targetterminal to this base station (step 505).

It should be noted that although the threshold value and the maximumallowable delay time are handled as being equivalent, it is permissibleto apply weighting. Further, in the manner described above, handover maybe performed from the terminal having lowest score. Furthermore,although handover is ranked according to priority, a terminal may beranked in order of decreasing priority.

Thus, in accordance with the fifth embodiment, effects similar to thoseof the first embodiment can be obtained.

(E) Sixth Embodiment

A sixth embodiment is one for controlling handover based upon throughputon a per-terminal basis.

FIG. 31 is a structural view of the sixth embodiment, and FIGS. 32, 33illustrate processing flows of a base station. FIG. 31 differs from thefirst embodiment of FIG. 1 in that a per-terminal throughput calculationunit 33 is provided instead of the per-QoS throughput calculation unit13, a required-throughput generator 34 is provided instead of thethroughput threshold value generator 14, and the handover controller 2 icontrols handover upon taking into consideration throughput on aper-terminal basis.

The six embodiment performs handover control upon calculating throughputwith respect to each terminal regardless of service quality QoS.

In accordance with the processing flow of FIG. 32, the throughputcalculation unit 33 measures throughput Tks2 of every terminal, withoutbeing aware of the service quality QoS of each terminal UEk, and inputsthe result to the handover controller 2 i through control similar tothat of the first embodiment (steps 601 to 605). Further, therequired-throughput generator 34 inputs the required throughput(stipulated throughput) Tks2 th of each terminal to the handovercontroller 2 i (step 606).

In response, the handover controller 2 i calculates dk2 in accordancewith the following equation using the throughput Tks2 and stipulatedthroughput threshold value Tks2 th (step 607):

$\delta_{k\; 2} = \frac{T_{{ks}\; 2} - T_{{ks}\; 2\; {th}}}{T_{{ks}\; 2{th}}}$

In accordance with this equation, the larger δk2, the more margin thereis with respect to the stipulated throughput, and the smaller δk2, theless margin. Similarly, the handover controller 2 i calculates δ12 toδm2 with respect to all of the terminals UE1 to UEm that have beenconnected to the base station (step 608). It should be noted that Tks2th may be the same value or a different value for all terminals.

Next, the handover controller 2 i continues processing in accordancewith the processing flow of FIG. 33. Specifically, the handovercontroller 2 i selects the minimum value of the aforesaid δ12 to δm2(step 651) and specifies this terminal UEn. That is, the handovercontroller 2 i selects the terminal UEk having the smallest margin withrespect to the stipulated throughput and performs handover givingprecedence to this terminal.

If a terminal to be handed over is found, the RNC is requested toperform handover. In response, the RNC selects a base station apparatusNode B, which has a different carrier frequency, as the handoverdestination (step 652) and exercises control to hand over the targetterminal to this base station (step 653).

Thus, in accordance with the sixth embodiment, effects similar to thoseof the first embodiment can be obtained.

(E) Seventh Embodiment

A seventh embodiment is one for controlling handover using necessarytransmission power.

FIG. 34 is a structural view of the seventh embodiment, FIG. 35 shows anexample of a protocol in the seventh embodiment, and FIGS. 36, 37illustrate processing flows of a base station. FIG. 35 differs from thefirst embodiment of FIG. 1 in that the block size setting unit 12,throughput calculation unit 13 and throughput threshold value generator14 are deleted, a transmission power calculation unit 41 and requiredtransmission power calculation unit 42 are provided instead, and theACK/NACK extraction unit 2 c is deleted. In the protocol of FIG. 35,processing P1, P5 is the same as that of the first embodiment butprocessing P21 is different from that of the first embodiment. FIGS. 36,37 illustrate the processing flows of processing P21.

In the first to third embodiments, it has been described that CQIindicates the reception environment (propagation environment) at acertain terminal. Here the CQI can be created based upon a transmissioncondition that satisfies a required error rate in a certain specificreception condition (e.g., modulation scheme, number of spreading codes,transmission power of the base station, etc.) (This is the case in the3GPP specifications.)

According to the seventh embodiment, control is carried out with regardto the transmission power of the base station.

In the CQI that has been sent from a certain UEk, as mentioned above,the required base-station transmission power for obtaining the requirederror rate in the terminal UEk becomes necessary. On the other hand,since the transmission power of the base station also is transmissionpower intended for other terminals, there is a stipulation on overalltransmission power. In other words, the total transmission power isdivided into transmission powers intended for each of the terminals. Inview of these two points, there is a possibility that a base-stationtransmission power for satisfying the required reception error rate of acertain terminal will be inadequate owing to a tradeoff with thetransmission powers intended for the other terminals. In case of such aninadequacy, an error may occur in reception at the terminal and aretransmission request may be sent back to the base station. In aworst-case scenario, there is the possibility that retransmission willbe repeated and, as a result, that the required throughput will not bemet.

In accordance with the processing flow of FIG. 36, if the requiredtransmission power calculation unit 42 receives the CQI of terminal UEk,then the unit calculates the base-station transmission power Pk,cqi formeeting the required reception error rate that conforms to this CQI andinputs the calculated power to the handover controller 2 i. Thebase-station transmission power Pk,cqi is capable of being found using apreviously prepared table of correspondence between CQI and Pk,cqi. Thetransmission power calculation unit 41 calculates transmission powerPk,s regarding the terminal UEk based upon an even balance relative tothe transmission powers that have been assigned to the other terminalsby the scheduler 2 h and inputs the calculated power to the handovercontroller 2 i (steps 701 to 704).

The handover controller 2 i obtains the difference between Pk,cqi andPk,s as a required power difference δk3 in accordance with the followingequation (step 705):

δk3=P _(k,cqi) −P _(k,s)≧0

The handover controller 2 i thenceforth calculates and stores δk3 withregard to all terminals (step 706).

If δk3 regarding all terminals is found, the handover controller 2 isubsequently selects the terminal, which has the maximum required powerdifference, as the handover terminal and requests the RNC for handoverin accordance with the processing flow of FIG. 37. The RNC selects thebase station apparatus Node B, which has a different carrier frequency,as the handover destination and exercises control to hand over thetarget terminal to this base station. It should be noted that handovermay be performed with respect to all terminals for which δk3<0 holds.

Thus, effects similar to those of the first embodiment can be obtainedby handover control of the seventh embodiment.

(H) Eighth Embodiment

FIG. 38 is a structural view of an eighth embodiment, and FIGS. 39, 40illustrate processing flows according to the eighth embodiment. FIG. 38differs from the first embodiment of FIG. 1 in that a base-stationthroughput calculation unit 51 and base-station required-throughputgenerator 52 are provided.

The eighth embodiment is such that even in a case where the throughputof each terminal connected to the base station Node B has sufficientmargin with respect to a throughput threshold value and it isunnecessary to perform handover with regard to each terminal, a handoverterminal is decided and handed over in the event that the throughput ofthe overall base station does not have enough margin with respect to thethroughput threshold value of the base station. It should be noted thatan instance in which it is not necessary to perform handover with regardto all terminals will be described for the sake of simplicity. However,there may be instances where handover control is necessary for someterminals.

At the base station Node Bk, the base-station throughput calculationunit 51 measures the throughput Tks at each terminal UEk, calculatesthroughput T_(k,NB) of the overall base station (the base-stationthroughput) based upon the average throughput of each terminal andinputs the result to the handover controller 2 i (step 801). Further,the base-station required-throughput generator 52 inputs the requiredthroughput (base-station throughput threshold value) T_(k,NBth) of thebase station to the handover controller 2 i (step 802).

Upon input of the base-station throughput and base-station throughputthreshold value thereto, the handover controller 2 i determines whetherthe base-station throughput T_(k,NB) meets the base-station throughputthreshold value T_(k,NBth) stipulated. That is, the handover controller2 i obtains the difference between these in accordance with thefollowing equation (step 803):

δ_(k4) =T _(k,NB) −T _(k,NBth)

and checks to determine whether δ_(k4)<0 holds (step 804).

If δ_(k4)≧0 holds, the handover controller 2 i decides not to implementhandover (step 805). On the other hand, if δ_(k4)<0 holds, the handovercontroller 2 i decides to implement handover (step 806) and, inaccordance with the first to seventh embodiments, decides the handoverterminal (step 807) and requests the RNC for handover (step 808). Inresponse, the RNC selects a base station apparatus Node B, which has adifferent carrier frequency, as the handover destination and exercisescontrol to hand over the target terminal to this base station.

The above-described processing makes it possible to improve thethroughput of the overall base station.

Further, consider two base stations Node B3 and Node B4 that cover thesame area and use separate frequencies as another method of handovercontrol. In accordance with the processing flow of FIG. 40, the RNCreceives throughputs T_(3,NB), T_(4,NB) of each of the base stations(steps 851, 852) and compares these (step 853). In case of an imbalancesuch that T_(3,NB)>>T_(4,NB) holds, or in other words, when one islarger than the other by more than a set value, handover is performedfrom one node, namely Node B3, to the other node, namely Node B4 (step854). Accordingly, handover from Node B4 to Node B3 is performed even ina case where T_(3,NB)<<T_(4,NB) holds. Handover is carried out using amethod of the kind illustrated in the first to seventh embodiments.However, if one is larger than the other by more than a set value,handover control is not carried out (step 855).

It should be noted that the RNC may gather information, make thesejudgments and execute processing, or information may be exchangedbetween base stations, judgments made and processing executed betweenthem.

By virtue of the control set forth above, an imbalance between thethroughputs of the two base stations can be corrected, an imbalance inbase-station loads can be reduced and load can be alleviated.

(I) Ninth Embodiment

A ninth embodiment decides whether to perform handover based upon numberterminals accommodated per base station. FIG. 41 is a structural view ofthe ninth embodiment, and FIGS. 42, 43 illustrate processing flowsaccording to the ninth embodiment. FIG. 41 differs from the firstembodiment of FIG. 1 in that an accommodated-terminal-number calculationunit 61 and accommodated-terminal-number threshold value generator 62are provided.

The ninth embodiment is such that even in a case where the throughput ofeach terminal connected to the base station Node B has sufficient marginwith respect to a throughput threshold value and it is unnecessary toperform handover with regard to each terminal, a handover terminal isdecided and handed over in the event that the number of terminalsaccommodated by the base station is large. It should be noted that aninstance in which it is not necessary to perform the handover of thefirst embodiment with regard to all terminals will be described for thesake of simplicity. However, there may be instances where handovercontrol is necessary for some terminals.

At the base station Node Bk, the accommodated-terminal-numbercalculation unit 61 calculates the number N_(UE,k) of accommodatedterminals and inputs the result to the handover controller 2 i (step901). Further, the accommodated-terminal-number threshold valuegenerator 62 inputs the threshold value N_(UEth) on the number ofterminals accommodated by the base station to the handover controller 2i (step 902). Upon input of the number N_(UE,k) of accommodatedterminals and threshold value N_(UEth) on the number of terminalsaccommodated thereto, the handover controller 2 i determines whether thenumber N_(UE,k) of accommodated terminals satisfies the stipulatedthreshold value N_(UEth) on the number of terminals accommodated. Thatis, the handover controller 2 i obtains the difference between these inaccordance with the following equation (step 903):

δ_(k5) =N _(UE,k) −N _(UEth)

and checks to determine whether δ_(k5)>0 holds (step 904).

If δ_(k5)≦0 holds, the handover controller 2 i decides not to implementhandover (step 905). On the other hand, if δ_(k5)>0 holds, the handovercontroller 2 i decides to implement handover (step 906) and, inaccordance with the first to seventh embodiments, decides the handoverterminal (step 907) and requests the RNC for handover (step 908). Inresponse, the RNC selects a base station apparatus Node B, which has adifferent carrier frequency, as the handover destination and exercisescontrol to hand over the target terminal to this base station.

The above-described processing makes it possible to reduce theprocessing load at the base station.

Further, consider two base stations Node B5 and Node B6 that cover thesame area and use separate frequencies as another method of handovercontrol. In accordance with the processing flow of FIG. 43, the RNCreceives numbers N_(UE,5), N_(UE,6) of terminals accommodated by eachbase station (steps 951, 952) and compares these (step 953). In case ofan imbalance such that N_(UE,5)>>N_(UE,6) holds, or in other words, whenone is larger than the other by more than a set value, handover isperformed from one node, namely Node B5, to the other node, namely NodeB6 (step 954). Accordingly, handover from Node B6 to Node B5 isperformed even in a case where N_(UE,5)<<N_(UE,6) holds. Handover iscarried out using a method of the kind illustrated in the first toseventh embodiments. However, if one is larger than the other by morethan a set value, handover control is not carried out (step 955).

By virtue of the control set forth above, an imbalance between thethroughputs of the two base stations can be corrected, an imbalance inbase-station loads can be reduced and load can be alleviated.

(J) Tenth Embodiment

A tenth embodiment implements handover to a different system. FIGS. 44and 45 are conceptual views, FIG. 46 illustrates a protocol and FIG. 47a processing flow.

In the first to ninth embodiments, handover is performed in the samesystem. In the tenth embodiment, however, it is assumed that thehandover destination is a different system (e.g., W-CDMA→PDC,W-CDMA→GSM).

In a case where the location at which the terminal UEk1 is situated isbeing serviced by a plurality of mobile communication systems (e.g.,W-CDMA and PDC, etc.) (FIG. 44), handover to a different system iscarried out.

Processing for deciding the handover terminal and handover control applythe first to ninth embodiments. This embodiment differs in that it isnecessary that handover be managed, controlled and implemented not onlyby the higher-order RNC 1 of the base station but between this and anRNC 1 that is at the handover destination. In other words, control isexercised in cooperation with the handover-destination RNC 1′ or basestation 2′ through a gateway 5, which is the point of connection betweenthe core network 4 constituting the higher layer of the RNC 1 and theother system (see FIG. 45).

Here the core network is constituted by, e.g., an MSC, GMSC, GGSN, etc.,and implements line switching control or packet switching control. Thecore network establishes a connection to a different system via thegateway. Further, the different system is a W-CDMA system, PDC system orGSM system, etc., of a different provider. Furthermore, it is requiredthat a terminal be capable of sending and receiving to and from aplurality of mobile communication systems. At present, a terminal ofthis kind is referred to as a dual-mode terminal. Terminals capable ofbeing used in W-CDMA and GSM are commercially available.

Processing P1 to P4 in the protocol of FIG. 46 is identical with that ofthe first embodiment; only the processing of processing P5 differs. Thatis, in the processing P5, handover to a different system is requested inaccordance with the processing flow shown in FIG. 47.

When implementation of handover is judged to be necessary at the RNC ornode B based the first to ninth embodiments, a request to implementhandover is sent to RNC 1′ or Node B2′ at the handover destination viathe core network 4, which is the higher layer, and the gateway 5 at thepoint of connection to the other system (steps 1001, 1002). The RNC 1′or Node B2′ at the handover destination that has received the requestchecks the possibility of handover implementation (step 1003) and, if itis found to be possible, requests the RNC 1 and Node B2 of thehandover-source system to perform handover. In response, handover to thedifferent system is performed using a different frequency or a differentfrequency and different modulation scheme (steps 1004, 1005).

By virtue of the foregoing, effects similar to those of the first toninth embodiments can be obtained. Another effect is that a service(QoS) that could not be accommodated in a certain system becomespossible in another system by movement to this system.

The following effects are produced by the present invention as set forthabove:

It is possible for a terminal to communicate without a failure such asloss of an image in the transmission of a moving picture.

A base station can reduce the processing load ascribable tocommunication. Further, it is possible to level the processing loadbetween base stations and between frequencies. This makes it possible toprovide services to new users.

Additional Note

1. A base station apparatus for wirelessly communicating with terminals,comprising:

means for monitoring a receiving state of each terminal;

detecting means for detecting, based upon the receiving state, aterminal in which there is a possibility that loss of a call or adecline in quality will occur or a terminal in which loss of a call or adecline in quality has occurred; and

control means for exercising control for handing over said terminal toanother base station apparatus having a different carrier frequency.

2. A mobile communication system having a terminal, a base stationapparatus for wirelessly communicating with said terminal, and a radionetwork controller for controlling the base station apparatus, wherein,

said base station apparatus comprises:

means for monitoring a receiving state of each terminal;

detecting means for detecting, based upon the receiving state, aterminal in which there is a possibility that loss of a call or adecline in quality will occur or a terminal in which loss of a call or adecline in quality has occurred; and

handover control means for requesting a radio network controller to handover said detected terminal to another base station apparatus; and

said radio network controller comprises:

means for handing over said terminal to another base station apparatus,which has a carrier frequency different from that of the present basestation apparatus, in response to said request.

3. A mobile communication system according to item 2, wherein said radionetwork controller performs handover to a different mobile communicationsystem.4. A handover method in a radio communication, comprising:

monitoring a receiving state of a terminal;

detecting, based upon the receiving state, a terminal in which there isa possibility that loss of a call or a decline in quality will occur ora terminal in which loss of a call or a decline in quality has occurred;and

exercising control for handing over said terminal to another basestation apparatus having a carrier frequency different from that of apresent base station apparatus.

5. A base station apparatus for wirelessly communicating with terminals,comprising:

a receiver/demodulator for receiving and demodulating a signal that hasbeen transmitted from a terminal;

an information extraction unit for extracting, from the demodulateddata, information indicative of the receiving state of the terminal andretransmission control information indicative of arrival/non-arrival ofdata;

a transmit buffer for holding transmit data and retransmit data on aper-terminal basis;

a transmit-data size setting unit for setting size of data, which istransmitted to said terminal, based upon the receiving state of saidterminal;

a transmit terminal selector for deciding, based upon the receivingstate of each terminal, to which terminal the data of said size is to betransmitted;

a transmitter for modulating and transmitting the data of said sizebeing held in said transmit buffer;

a transmission speed calculation unit for calculating transmission speedbased upon size of said transmit data and time of transmission and timeof reception of said data; and

handover control means for exercising control, if a terminal requireshandover based upon a required transmission speed, which is decided byservice quality of said transmit data, and the calculated transmissionspeed, in such a manner that said terminal is handed over to anotherbase station apparatus having a different carrier frequency.

6. A base station apparatus according to item 5, further comprising,means for storing a threshold value of transmission speed on aper-service-quality basis; wherein

said handover controller decides a handover terminal based upon size ofa difference between a speed margin value, which is the differencebetween said calculated transmission speed and said requiredtransmission speed, and said speed threshold value.

7. A base station apparatus according to item 5, wherein said handovercontrol means judges the necessity of handover with regard to eachterminal based upon said required transmission speed and said calculatedtransmission speed, and requests a radio network controller to performhandover if handover is necessary and said radio network controllerexercised control for handing over said terminal to another base stationapparatus, which has a carrier frequency different from that of thepresent base station apparatus.8. A base station apparatus according to item 5, wherein said handovercontrol means inputs, on a per-terminal basis, the required transmissionspeed decided by service quality of said transmit data and saidcalculated transmission speed to a radio network controller; and

said radio network controller judges the necessity of handover withregard to a terminal based upon said required transmission speed andsaid transmission speed received from the base station apparatus and, ifhandover is necessary, performs handover to another base stationapparatus having a carrier frequency different from that of the presentbase station apparatus.

9. A base station apparatus according to item 5, further comprising:

priority setting means for setting order of priority between amongterminals, order of priority among service qualities or order ofpriority among receiving states of terminals; and

means for deciding a first order of priority of handover based upon theterminal, or service quality of transmit data or receiving quality ofthe terminal, and a second order of priority of handover based upon adifference between said required transmission speed and said calculatedtransmission speed; wherein

said handover control means judges the necessity of handover with regardto each terminal based upon the first and second orders of priority, andhands over said terminal to another base station apparatus having adifferent carrier frequency if handover is necessary.

10. A handover method in a mobile communication system having terminals,a base station apparatus for wirelessly communicating with saidterminals, and a radio network controller for controlling the basestation apparatus, comprising:

receiving and demodulating a signal that has been transmitted from eachterminal;

extracting CQI information, which is indicative of the receiving stateof the terminal, from the demodulated data;

setting size of data, which is transmitted to said terminal, based uponthe CQI information of said terminal;

calculating transmission speed based upon size of said transmit data andtime of transmission and time of reception of said data; and

judging whether handover is necessary with regard to each terminal basedupon a required transmission speed, which is decided by service qualityof said transmit data, and said calculated transmission speed, andhanding over said terminal to another base station apparatus having adifferent carrier frequency if handover is necessary.

11. A handover method according to item 10, further comprising:

storing beforehand a threshold value of transmission speed on aper-service-quality basis; and

deciding a handover terminal based upon size of a difference between aspeed margin value, which is the difference between said calculatedtransmission speed and said required transmission speed, and said speedthreshold value.

12. A base station apparatus for wirelessly communicating withterminals, comprising:

a receiver/demodulator for receiving and demodulating a signal that hasbeen transmitted from a terminal;

an information extraction unit for extracting, from the demodulateddata, information indicative of the receiving state of the terminal andretransmission control information indicative of arrival/non-arrival ofdata;

a transmit buffer for holding transmit data and retransmit data on aper-terminal basis;

a transmit-data size setting unit for setting size of data, which istransmitted to said terminal, based upon the receiving state of saidterminal;

a transmit terminal selector for deciding, based upon the transmittingstate of each terminal, to which terminal the data of said size is to betransmitted;

a transmitter for modulating and transmitting the data of said sizebeing held in said transmit buffer;

a transmission delay time calculation unit for calculating transmissiondelay time based upon size of said transmit data and time oftransmission and time of reception of said data; and

handover control means for judging the necessity of handover with regardto each terminal based upon maximum allowable delay time, which isdecided by service quality of said transmit data, and said calculatedtransmission delay time, and exercising control to hand over saidterminal to another base station apparatus having a different carrierfrequency if handover is necessary.

13. A base station apparatus according to item 12, wherein said handovercontrol means judges the necessity of handover with regard to eachterminal based upon said maximum allowable delay time and saidcalculated transmission delay time, and requests a radio networkcontroller to perform handover if handover is necessary and said radionetwork controller exercises control for handing over said terminal toanother base station apparatus, which has a carrier frequency differentfrom that of the present base station apparatus.14. A base station apparatus according to item 12, wherein said handovercontrol means inputs, on a per-terminal basis, said maximum allowabledelay time and said calculated transmission delay time to a radionetwork controller; and

said radio network controller judges the necessity of handover withregard to a terminal based upon said maximum allowable delay timereceived from the base station apparatus and said calculatedtransmission delay time received from the base station apparatus, andperforms handover to another base station apparatus having a carrierfrequency different from that of the present base station apparatus ifhandover is necessary.

15. A base station apparatus according to item 12, further comprising:

a transmission speed calculation unit for calculating transmission speedbased upon size of said transmit data and transmission time andreception time of said data; and

means for deciding a first order of priority of handover based upon arequired transmission speed, which is decided by the service quality ofthe transmit data, and said calculated transmission speed, and a secondorder of priority of handover based upon said maximum allowable delaytime and said calculated transmission delay time; wherein

said handover control means judges the necessity of handover with regardto each terminal based upon the first and second orders of priority, andhands over said terminal to another base station apparatus having adifferent carrier frequency if handover is necessary.

16. A handover method in a mobile communication system having terminals,a base station apparatus for wirelessly communicating with saidterminals, and a radio network controller for controlling the basestation apparatus, comprising:

receiving and demodulating a signal that has been transmitted from eachterminal;

extracting CQI information, which is indicative of the receiving stateof the terminal, from the demodulated data;

setting size of data, which is transmitted to said terminal, based uponthe CQI information of said terminal;

calculating transmission delay time based upon size of said transmitdata and time of transmission and time of reception of said data; and

judging whether handover is necessary with regard to each terminal basedupon a maximum allowable delay time, which is decided by service qualityof said transmit data, and said calculated transmission delay time, andhanding over said terminal to another base station apparatus having adifferent carrier frequency if handover is necessary.

17. A base station apparatus for wirelessly communicating withterminals, comprising:

a receiver/demodulator for receiving and demodulating a signal that hasbeen transmitted from a terminal;

an information extraction unit for extracting, from the demodulateddata, information indicative of the receiving state of the terminal andretransmission control information indicative of arrival/non-arrival ofdata;

a transmit buffer for holding transmit data and retransmit data on aper-terminal basis;

a transmit-data size setting unit for setting size of data, which istransmitted to said terminal, based upon the receiving state of saidterminal;

a transmit terminal selector for deciding, based upon the transmittingstate of each terminal, to which terminal the data of said size is to betransmitted;

a transmitter for modulating and transmitting the data of said sizebeing held in said transmit buffer;

a transmission speed calculation unit for calculating transmission speedbased upon size of said transmit data and time of transmission and timeof reception of said data; and

judging the necessity of handover with regard to each terminal basedupon a difference between said calculated transmission speed and atransmission speed threshold value, and requesting a radio networkcontroller to perform handover if handover is necessary; said terminalbeing handed over to another base station apparatus, which has a carrierfrequency different from that of the present base station apparatus, bysaid radio network controller.

18. A base station apparatus for wirelessly communicating withterminals, comprising:

a receiver/demodulator for receiving and demodulating a signal that hasbeen transmitted from a terminal;

an information extraction unit for extracting, from the demodulateddata, information indicative of the receiving state of the terminal;

a transmit buffer for holding transmit data and retransmit data on aper-terminal basis;

a transmitter for modulating and transmitting the data being held insaid transmit buffer;

a required transmission power calculation unit for calculating arequired transmission power, which is necessary in order to maintainsaid terminal at a transmission quality above a fixed quality, basedupon the information indicative of the receiving state of said terminal;and

a handover controller for judging the necessity of handover based uponactual transmission power and the required transmission power, andrequesting a radio network controller to perform handover if handover isnecessary; wherein

said terminal is handed over to another base station apparatus, whichhas a carrier frequency different from that of the present base stationapparatus, by said radio network controller.

19. A base station apparatus for wirelessly communicating withterminals, comprising:

a receiver/demodulator for receiving and demodulating a signal that hasbeen transmitted from a terminal;

an information extraction unit for extracting, from the demodulateddata, information indicative of the receiving state of the terminal andretransmission control information indicative of arrival/non-arrival ofdata;

a transmit buffer for holding transmit data and retransmit data on aper-terminal basis;

a transmit-data size setting unit for setting size of data, which istransmitted to said terminal, based upon the receiving state of saidterminal;

a transmit terminal selector for deciding, based upon the transmittingstate of each terminal, to which terminal the data of said size is to betransmitted;

a transmitter for modulating and transmitting the data of said sizebeing held in said transmit buffer;

a transmission speed calculation unit for calculating transmission speedbased upon size of said transmit data and time of transmission and timeof reception of said data;

a base station transmission speed calculation unit for calculatingtransmission speed of the overall base station using said transmissionspeed calculated with regard to each terminal; and

a handover controller for judging the necessity of handover based uponrequired transmission speed of the overall base station and saidcalculated base station transmission speed and, if handover isnecessary, deciding to which terminal handover is to be performed basedupon required transmission speed, which is decided by service quality oftransmit data, and said calculated transmission speed, and requesting aradio network controller to perform handover; wherein

said terminal is handed over to another base station apparatus, whichhas a carrier frequency different from that of the present base stationapparatus, by said radio network controller.

20. A base station apparatus according to item 19, further comprisingmeans for comparing base station transmission speeds of neighboring basestation apparatus and, if the difference is greater than a set value,handing over a terminal that is under the control of a base stationhaving a high base station transmission speed to another neighboringbase station apparatus.21. A base station apparatus for wirelessly communicating withterminals, comprising:

a receiver/demodulator for receiving and demodulating a signal that hasbeen transmitted from a terminal;

an information extraction unit for extracting, from the demodulateddata, information indicative of the receiving state of the terminal andretransmission control information indicative of arrival/non-arrival ofdata;

a transmit buffer for holding transmit data and retransmit data on aper-terminal basis;

a transmit-data size setting unit for setting size of data, which istransmitted to said terminal, based upon the receiving state of saidterminal;

a transmit terminal selector for deciding, based upon the transmittingstate of each terminal, to which terminal the data of said size is to betransmitted;

a transmitter for modulating and transmitting the data of said sizebeing held in said transmit buffer;

a transmission speed calculation unit for calculating transmission speedbased upon size of said transmit data and time of transmission and timeof reception of said data;

an accommodated-terminal-number calculation unit for calculating numberof terminals accommodated by a base station; and

a handover controller for judging the necessity of handover based uponan accommodated-terminal-number threshold value, which is a thresholdvalue on number of terminals accommodated by the base station, and saidcalculated number of terminals accommodated and, if handover isnecessary, deciding to which terminal handover is to be performed basedupon required transmission speed, which is decided by service quality oftransmit data, and said calculated transmission speed, and requesting aradio network controller to perform handover; wherein

said terminal is handed over to another base station apparatus, whichhas a carrier frequency different from that of the present base stationapparatus, by said radio network controller.

22. A base station apparatus according to item 21, further comprisingmeans for comparing numbers of terminals accommodated by neighboringbase station apparatuses and, if the difference is greater than a setvalue, handing over a terminal that is under the control of a basestation having a large number of accommodated terminals to anotherneighboring base station apparatus.

1. A mobile communication system having a terminal, a base stationapparatus for wirelessly communicating with said terminal, and a radionetwork controller for controlling the base station apparatus, wherein,said base station apparatus comprises: a monitoring portion configuredto monitor transmission speed of each terminal; a detecting portionconfigured to detect, based upon the transmission speed, a terminal inwhich there is a possibility that loss of a call or a decline in qualitywill occur; and a handover control portion configured to request a radionetwork controller to hand over said detected terminal to another basestation apparatus; and said radio network controller comprises: aportion configured to hand over said terminal to another base stationapparatus, which has a carrier frequency different from that of thepresent base station apparatus, in response to said request.
 2. A mobilecommunication system according to claim 1, wherein said radio networkcontroller performs handover to a different mobile communication system.3. A base station apparatus for wirelessly communicating with aterminal, comprising: a monitoring portion configured to monitortransmission speed of each terminal; a detecting portion configured todetect, based upon the transmission speed, a terminal in which there isa possibility that loss of a call or a decline in quality will occur;and a handover control portion configured to request a radio networkcontroller to hand over said detected terminal to another base stationapparatus wherein said radio network controller hands over said terminalto another base station apparatus, which has a carrier frequencydifferent from that of the present base station apparatus, in responseto said request.